1. Field of the Invention
The present invention relates to an optical unit for use in a liquid crystal display (LCD) device, and to a LCD device incorporating such optical unit.
2. Description of the Related Art
In the field concerning miniaturized LCD devices, use of an optical unit as a built-in backlight has widely spread. Commonly, such optical unit includes a light conductive plate and a source of light, for example, a fluorescent tube or LED. The incident light from the source of light impinges upon the adjacent edge of the light conductive plate. The light diffuses within the light conductive plate. The diffused light illuminates the associated LCD through a planar surface of the light conductive plate. A light reflective sheet covers at least a portion of the light conductive plate to reflect the diffused light back inwardly. This so-called edge type system is advantageous in thinness.
One example of the edge type system is illustrated in FIGS. 11 and 12. FIG. 11 is a plan view of an optical unit 120 according to the prior art with unnecessary sheets removed to illustrate a frame 102. FIG. 12 is a cross section taken through the line 12-12 in FIG. 11. The frame 102 is a rectangular receptacle with a shallow rectangular space. The frame 102 includes four-sided inner peripheral wall and an end wall, which are interconnected to define the shallow rectangular space. As best seen in FIG. 12, the frame 102 receives within the shallow rectangular space a reflective sheet 103 covering the end wall, a light conductive plate 101, and a LED 113. The frame 102 has two positioning grooves 112, with which the opposed sides of the inner peripheral wall are formed, respectively. The light conductive plate 101 has two positioning projections 111 inserted into the positioning grooves 112, respectively. The two projections 111 extend outwardly from sides of the light conductive plate 101. The light conductive plate 101 has a surface covered with a lens sheet 104 and a diffusion sheet 105. The lens sheet 104 extends over almost the entire area of the surface of the light conductive plate 101. The diffusion sheet 105 covers the entire area of the surface of the lens sheet 104. An antiglare sheet 107, which is formed with a rectangular opening, is fast on the frame 102 by adhesive 106 in a manner to cross the boundary between the inner peripheral wall of the frame 102 and the light conductive plate 101 (see FIG. 12).
The market for mobile terminals is growing bigger and bigger. Personal digital assistants (PDA) and mobile telephones constitute the market. In the market, there is strong demand for a reduction in thickness, weight and cost of LCD devices for beneficial application to the PDA and mobile telephones. Accordingly, a need remains for a thin, light and inexpensive optical unit.
The illustrated known optical unit 120 does not meet this need to a satisfactory level. In this known unit, the end wall of the frame 102 plays an important role in retaining an appropriate relationship between the light conductive plate 101 and light reflective plate 103.
With regard to the relationship between a light conductive plate and a frame, JP-A 11-305228 proposes interposing a light conductive plate between a frame and a light reflective sheet and adhering the light reflective plate to the frame. Without the light reflective plate, the light conductive plate would drop out of the frame because it is not held stationary relative to the frame. The light conductive plate is fast on the frame by adhesive only. Thus, there is a potential problem that the appropriate positional relationship between the light conductive plate and the frame may not hold if the light conductive plate is stressed against the light reflective sheet.